Human skin comprises three principal layers: the epidermis, the dermis, and the subcutaneous fat layer. The epidermis comprises four layers (from top to bottom): the stratum corneum, the granular layer, the spiny layer, and the basal layer. A separate fifth layer, the stratum lucidum, may be present between the stratum corneum and granular layer. The basal layer produces cells which gradually migrate upward to form the other epidermal layers. As these cells migrate upward, they lose their central nucleus and start to produce skin proteins (keratins) and fats (lipids). These cells are known as keratinocytes when present in the upper layers of the epidermis. Melanocytes are another class of cells located in the basal layer of the epidermis. Melanocytes are responsible for the production of melanin, a pigment which is a primary factor in the tonal appearance of skin.
Referring to FIGS. 1, 2, and 3, melanin is produced by a complex set of reactions within the melanocyte involving, at a basic level, the enzyme tyrosinase and L-tyrosine as a substrate. Tyrosinase catalyzes the conversion of L-tyrosine to DOPA (L-3,4-dihydroxyphenylalanine) and of DOPA to dopaquinone. Dopaquinone undergoes further conversion to form melanin. Melanin aggregates in organelles known as the melanosomes which are transferred to keratinocytes along slender filaments of the melanocyte known as dendrites. The production of tyrosinase and its activity determine, in part, the amount of melanin produced. The amount and the type of melanin transferred to the keratinocytes determine, for their part, the degree of visual pigmentation of human skin.
One mechanism in the melanin production cycle is the transfer of melanosomes from the melanocytes to the keratinocytes by way of phagocytosis. Research has found that the protease-activated receptor 2 (PAR-2) expressed on keratinocytes is involved in melanosome transfer and therefore may regulate pigmentation. See, e.g., Seiberg, M. et al., The Protease-Activated Receptor 2 Regulates Pigmentation via Keratinocyte-Melanocyte Interactions, Experimental Cell Research 254, 25-32 (2000). Activation of PAR-2 with trypsin (or a trypsin-like protease) (or with the peptide agonist SLIGRL) initiates melanosome transfer, thereby contributing to the distribution of melanin within keratinocytes. In some cases, the distribution of melanin may manifest as an age spot or uneven skin tone. Compounds that inhibit trypsin (or a trypsin-like protease) activation of PAR-2 are believed to disrupt or reduce the phagocytosis of the melanocytes by the keratinocytes. Compounds that inhibit the PAR-2 either by inhibiting trypsin activity or by inhibiting SLIGRL peptide binding may regulate hyperpigmentation and melanin overproduction. See, e.g., Seiberg, M. et al., The Protease-Activated Receptor 2 Regulates Pigmentation via Keratinocyte-Melanocyte Interactions, Experimental Cell Research 254, 25-32 (2000).
In young skin, melanin is evenly distributed, and melanocyte activity is low, restricted to the production of constitutive pigmentation only. In aging skin, overzealous melanogenesis production and subsequent melanin transport can eventually create permanent local discoloration with sufficient size and contrast to appear as age spots (lentigines) or as diffuse hyperpigmentation or an uneven skin tone. Compositions and methods of treatment that minimize, reduce, ameliorate, or treat the size and/or contrast of age spots and/or which improve overall skin tone are continuing desires in the cosmetic field.